Recently, there have been proposed a large number of systems in which waste produced in cities is disposed of by means of thermal decomposition. Further, there have been proposed a large number of shaft furnaces in which the residue of thermal decomposition is melted so as to decrease the volume of residue and further decrease the quantity of hazardous components. Japanese Examined Patent Publications (Kokoku) No. 46-34349 and No. 52-24790 propose the leading technology in this technique. Referring to FIG. 1, the melting type thermal decomposing waste disposal system disclosed by the above patent publications will be explained below. FIG. 1 is a view showing a reaction in the burning and melting furnace. Waste and coke are charged into the shaft section la from the input port (charging port) 11 arranged on the furnace top. While the thus charged waste and coke are forming charge layers (packing layers) in the furnace, they move down successively. While they are moving down, waste is successively melted by the action of reaction gas (thermally decomposed gas) that rises upward in the furnace. Exhaust gas produced in this treatment is transferred from the exhaust gas pipe (duct) 12, arranged adjacent to the charging port, to an exhaust gas treatment apparatus not shown in the drawing. In this exhaust gas treatment apparatus, heat in the exhaust gas is recovered and the exhaust gas is subjected to gas treatment. Specifically, trash that has been charged into the furnace from the input port 11 accumulates in the shaft section 1a. Water is removed from the waste in the uppermost section 6 of the shaft section 1a. The lower shaft section and the bosh section 5 compose a thermal decomposing zone 7. In the lower section of the thermal decomposing zone, there exists a large quantity of char which is made of carbide. Carbonic massive combustibles, which will be referred to as coke in this specification hereinafter, are burned by the action of a burning support gas that has been fed from tuyeres. This portion of the furnace in which the combustibles are burned is referred to as a burning and melting zone 8. Since the melted slag and iron are reserved in the basin 13, they are intermittently discharged from the slag discharge holes (not shown in the drawing), which are arranged close to the basin 13, when the slag discharge holes are opened by drills. After slag has been discharged from the furnace, the slag discharge holes are closed with mortar or the like. Reference numeral 10 is called a hearth section as a general term when it is compared with the reference numerals 5 and 1a. Exhaust gas produced in the process of burning and thermal decomposition is discharged from the duct 12.
After waste has been charged into the furnace, it is treated in the three steps of drying, thermal decomposition and burning melting. If the waste is not sufficiently dried, waste containing a large quantity of water enters the burning and melting zone. As a result, temperature in the burning and melting zone is remarkably lowered. Accordingly, it is necessary to simultaneously provide the following two functions. One is to provide a sufficiently large quantity of heat to dry the waste charged into the furnace, and the other is to provide a sufficiently large quantity of heat to melt the waste so as to occur the pyrogenetic reaction.
In this connection, an example of waste produced in a city contains combustibles 47.4%, garbage 26.6%, rubber and plastics 8.0%, and incombustibles 18.0% (wet base). In this case, the water content in waste is 40 to 60%. Accordingly, when the melting type thermal decomposing furnace is used, it is necessary to remove water from waste for the above reasons.
According to the technique disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-11766 which is the prior art in this technical field, it is impossible to stably operate the burning and melting furnace in the case of a change in the composition of the usual waste, wide area disposal waste, reprocessing incineration residue from other incineration equipment, or mixing process with reclaimed waste, sludge or the like in the same furnace.
The reasons are as follows. When the ratio of either water or incombustibles is high, it is necessary to increase the quantity of coke which is an auxiliary fuel, and further it is necessary to increase a quantity of oxygen gas to be fed from the lower tuyeres in accordance with the increase in the quantity of coke. On the contrary, when a ratio of combustibles is high, in order to operate the furnace stably and effectively, it is necessary to decrease a quantity of coke and increase a quantity of waste to be burned by the upper tuyeres.
According to Japanese Examined Patent Publication (Kokoku) Nos. 63-3207 and 2-606, there has been disclosed a method in which a quantity of carbonic combustibles to be fed into the furnace and a quantity of gas containing oxygen to be fed onto the hearth of high temperature are controlled in order to maintain the oxygen concentration in an upper portion of the charged layer to be approximately zero and also maintain the exhaust gas temperature to be 700 to 1200.degree. C. Also, there is disclosed a method in which a quantity of waste to be charged into the furnace is controlled in order to maintain the exhaust gas temperature in an upper portion of the charged layer to be 400 to 1200.degree. C.
In the former case, in which the upper tuyeres are arranged, it is impossible to apply the method as it is. That is, it is impossible to operate the furnace stably only when a quantity of carbonic combustibles and a quantity of gas containing oxygen to be fed onto the hearth of high temperature are controlled. (According to the present invention, gas containing oxygen is fed onto the hearth of high temperature from the lower tuyeres.) The reason why it is impossible to attain a stable operation of the furnace is described as follows. When only the quantity of carbonic combustibles and the quantity of oxygen gas fed from the lower tuyeres are controlled, only the burning and melting condition is changed. Therefore, in the case of waste, the quantity of combustibles of which is large, it is impossible to maintain a quantity of waste to be burned by the upper tuyeres at an appropriate value. Accordingly, a stable operation can not be carried out.
In the latter case, in which the packed layers in the drying zone, the thermal decomposing zone and the burning and melting zone are formed and the upper tuyeres are arranged, it is impossible to apply the method as it is. The quantity of waste to be fed is determined by the operating speeds in the drying zone, the thermal decomposing zone and the burning and melting zone. Therefore, unless the quantity of waste burned by the upper tuyeres is properly controlled, it is impossible to adjust the temperature of exhaust gas.